Examples of carbon protective layers used for coating sliding-resistant members or friction-resistant members include diamond-like carbon (DLC) films and graphite carbon films. Carbon protective layers are formed on magnetic recording layers of magnetic recording media to serve as sliding-resistant members to protect the magnetic recording layers from damage or corrosion induced by sliding and contact with magnetic heads. In recent years, DLC films formed by a plasma CVD method have been used. This is because a DLC film formed by the plasma CVD method is denser and harder than a film formed by a sputtering method. This feature is apparently due to the fact that the DLC film formed by the plasma CVD method is formed by hydrocarbon radicals, and a tetrahedral structure with a high three-dimensional rigidity is easily formed via hydrogen.
The increase in recording density is a requirement applied to magnetic recording media, and reducing the thickness of protective film and decreasing the distance between a head element and a magnetic recording layer is an effective means for meeting this requirement. When the film thickness is reduced, it is important to have a uniform in-plane distribution of the film thickness. Thus, even where the in-plane average value of film thickness is sufficient to obtain the predetermined electromagnetic conversion characteristic and the thickness is sufficient to ensure reliability, such as corrosion resistance, where the in-plane distribution of film thickness is nonuniform, the predetermined electromagnetic conversion characteristic cannot be obtained in thick places, whereas the reliability such as corrosion resistance cannot be ensured in thin places.
A method of disposing an appropriate shield within a plasma CVD chamber when a DLC film is formed by a plasma CVD method with the object of obtaining a uniform in-plane distribution of protective film thickness has been investigated. See for example Japanese Patent Applications Laid-open Nos. 2001-148118 and 2003-30823. With the usual plasma CVD method, the film thickness in the central zone of a substrate for film formation tends to increase because plasma irradiating the substrate is concentrated in the central zone. Disposing a shield in the vicinity of the central zone can make the film thickness more uniform. See, for example, paragraph 0010 of Japanese Patent Application Laid-open No. 2003-30823.
Further improvements, however, are needed to obtain a more uniform film thickness. Accordingly, there still remains a need for forming a carbon protective layer using a plasma CVD method, which makes it possible to bring the in-plane distribution of film thickness close to a uniform distribution.